The present invention relates to a heat roller fixing device, and more particularly to a heat roller fixing device which, as shown in FIG. 1, comprises a heating roller 5 incorporating a heater 4 and having a fluorine resin layer 3 over an electrically conductive core member 1 with a primer layer 2 interposed therebetween, and a pressure roller 8 having an insulating layer 7 over an electrically conductive core member 6 and adapted for pressing contact with the heating roller 5. When the fixing device is in operation, the heating roller 5 is heated by the heater 4 at a suitable temperature of 140.degree. to 180.degree. C. in accordance with the kind of toner used. Copy paper P having a charged toner image 9 formed thereon is passed between the rollers 5 and 8 with the image bearing surface facing the heating roller 5, whereby the toner image 9 is fixed onto the copy paper P. The drawing further shows a guide plate 10, a discharge roller 11, a separating pawl 12 and a thermistor 13.
With heat roller fixing devices of this type, the surface of the heating roller has good release properties sufficient to prevent thermal adhesion of the toner without necessitating application of silicone oil or like offset preventing agent, or with use of only a very small amount of such agent. The device therefore renders the copy paper free from stains and gives prints which feel good to the touch. However, the fluorine resin surface layer 3 of the heating roller 5, which is triboelectrically charged by the insulating surface layer 7 of the pressure roller 8 or by the copy paper P, is liable to electrostatically attract toner images and to permit offset, whereby: the offset preventing agent is unable to preclude the offset due to triboelectrification.
The present inventor, together with other inventors, has already provided a heat roller fixing device wherein the primer layer 2 of the heating roller 5 is formed of a fluorine resin containing carbon black and which is adapted to prevent the offset due to such triboelectrification (U.S. patent application Ser. No. 351,930 filed on Feb. 24, 1982). This device solved a problem conventionally experienced. With the known device shown in FIG. 1, the triboelectric charges on the heating roller 5, which were of negative polarity when the assembly started passage of paper, reached a maximum of more than -600 V upon lapse of about 5 minutes, Then reversed to positive polarity upon lapse of about 10 minutes and thereafter reached about +200 V max. as illustrated in FIG. 9 by hatching, whereas negative charging is almost avoidable with the device of U.S. Ser. No. 351,930. Accordingly the heating roller 5, is readily held at positive; polarity to inhibit the electrostatic attraction of toner to the roller 5 and prevent offset when the toner is charged positively.
Nevertheless, since it is nearly impossible to eliminate the positive charges, offset is unavoidable when a toner of negative polarity is used, so that the device is limited in the polarity of the toner to be used, since the charges could produce an adverse effect.